Piece has essential parts for voice and choir introduced by speaker (8)
I believe the answer is:
oratorio
'piece' is the definition.
(I have seen 'piece of music' mean 'oratorio' so perhaps 'piece' could also mean 'oratorio')
'choir introduced by speaker' is the wordplay.
'choir' becomes 'io' (I am not sure about this - if you are sure you should believe this answer much more).
'introduced by' says to put letters next to each other.
'speaker' becomes 'orator' (orator is a kind of speaker).
'io' after 'orator' is 'ORATORIO'.
'has essential parts for voice and' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for oratorio that I've seen before include "Eg, The Seasons (Haydn)" , "Musical work for orchestra and voices" , "Noted work" , "Work for voices, chorus and orchestra with biblical theme" , "Maybe Elijah" .)